1. Field of the Invention
The present invention relates to an oxygen reduction electrode and a fuel cell including the same.
2. Description of the Related Art
A fuel cell is an electrochemical device which directly transforms the chemical energy of hydrogen and oxygen which are contained in hydrocarbon materials such as methanol, ethanol, and natural gas into electrical energy. The energy transformation process of a fuel cell is very efficient and environmentally-friendly, thereby drawing attention for the past few years.
Fuel cells can be classified into Phosphoric Acid Fuel Cells (PAFCs), Molten Carbonate Fuel Cells (MCFCs), Solid Oxide Full Cells (SOFCs), Polymer Electrolyte Membrane Fuel Cells (PEMFCs), and Alkaline Full Cells (AFCs) according to the type of electrolyte used. All fuel cells operate on the same principle, but the type of fuel used, operating speed, the catalyst used and the electrolyte used are different. In particular, PEMFCs can be used in small-sized stationary power generation equipment or transportation systems due to their high reaction speed, low operating temperature, high output density, rapid start-up, and variation in requested output.
A fuel cell generally includes two electrodes, that is, a cathode and an anode, and a polymer electrolyte membrane. The cathode and the anode are generally each formed of a current collector, a gas diffusion layer, and a catalyst layer. A proton is dissociated from hydrogen or fuel, such as methanol, supplied to the anode (H2->2H++2e, E=0.000 V). Next, the proton is delivered to the cathode, which joins with oxygen to form water (O2+4H++4e->2H2O, E=1.229V). The cathode is also called an oxygen reduction electrode because an electric chemical reaction, where oxygen combines with the proton, is generated.
As described above, an oxygen reduction reaction generally follows a triple phase boundary reaction below on the surface of a catalyst.

In the oxygen reduction reaction, oxygen supplied in a gaseous state from the atmosphere transforms into liquid in a catalyst layer, and then reacts with a proton inside an electrolyte near the surface of a catalyst. Accordingly, the solubility of oxygen is an important factor in the operating temperature of a fuel cell, reaching about 150° C.
FIG. 1 is a schematic drawing of an operating mechanism of an oxygen reduction electrode when the porosity of a catalyst layer is maintained. Referring to FIG. 1, when the porosity of a catalyst layer is maintained, oxygen supplied in a gaseous state can penetrate deeply into the catalyst layer. Accordingly, oxygen is enabled to sufficiently contact the catalyst layer, thus increasing the solubility of oxygen. However, as shown in FIG. 2, when porosity of a catalyst layer is blocked by an electrolyte, flooding is generated. Accordingly, oxygen cannot penetrate into the catalyst layer, thus decreasing the solubility of oxygen. Thus, low oxygen concentration has adverse effects on the performance of a cell.